Liquid Crystal Display (LCD) devices have become increasingly important in displays which require very low consumption of electrical power or where the environment dictates a lightweight, planar, flat surface. Thus LCD's are used in display technologies such as wristwatches, personal computers, aircraft cockpit displays, etc.
In the simplest and most advantageously manufactured form, a field effect LCD device is prepared causing alignment of liquid crystal molecule to occur at a certain angle, the tilt angle, with respect to the plane of the inside of the glass plates. The inside of these glass plates of the LCD device have coatings of sets of electrodes (electrical conductors), usually judicious ratios of indium-tin oxide (ITO) or tin oxide. The sets of electrodes are usually etched conductors into the pattern compatible with the information to be displayed by the LCD. The alignment process is most easily carried out by solution casting (spin coating, roll coating, dipping, spraying, and/or doctor blading) a mixture of various organic polymers onto two glass/ITO substrates. After judicious removal of solvents and/or curing of the polymer layers, the glass plates are usually rubbed or buffed in one direction with cloths. The rubbing process serves to establish a unique optical direction. After rubbing both plates, these plates are rotated with respect to each other (the rotations being in the range from 90-270 degrees), adhered together (using either organic or inorganic adhesives) in such a way as to preserve a constant thickness to a space or gap between the plates, filled with various mixtures of LC molecules, and finally sealed by either organic or inorganic adhesives. At this stage polarizing films are often attached by a lamination process. Finally, electrical connections are made to both plates in a manner consistent with the electrical and display designs.
The use of rubbed polymer films (tilt angle controlling films) dominates the process technology in the production of all categories of nematic Liquid Crystal Displays (LCD's), and polyimides are the most common alignment films in use today. However, the exact relationship between the physical properties of the polyimide surfaces, the rubbing process, and the intrinsic properties of the Liquid Crystal mixtures are poorly understood. Recent works (M. E. Becker, R. A. Kilian, B. B. Kosmowski and D. A. Mlynski (1986); Alignment Properties of Rubbed Polymer Surfaces; Mol. Cryst. Liq. Cryst., 132, 167-180) have shown that tilt bias angle (TBA) reaches a constant limiting value as the rubbing energy increases, the polyimide and the Liquid Crystal mixture being held constant. It is also known that tilt angles vary over wide ranges (0 to 90 degrees) depending on the composition of certain coupling agents (J. Cognard (1982); Alignment of Nematic Liquid Crystals and their Mixtures: Mol. Cryst. Liq. Cryst., Suppl. 1, 1-75; also R. W. Filas and J. S. Patel (1987). Chemically Induced High-Tilt Surfaces for Liquid Crystals; Appl. Phys. Lett., 50(20), 1426-1428).
By combining some of the properties of coupling agents with the nature of certain polymer backbones, one can control the tilt angle by either chain termination or by drastic increases in polymer modulus However, these materials lack in general a wide process latitude. Such examples are given by
J. Nehring, H. Amstutz, P. A. Holmes and A. Nevin (1987); High-Pre-tilt Polyphenylene Layers for Liquid-Crystal Displays; Appl. Phys. Lett., 51(16), 1283-1284; and also PA0 H. Fukuro and S. Kobayashi (1988); Newly Synthesized Polyimide for Aligning Nematic Liquid Crystals Accompanying High Pretilt Angles; Mol. Cryst. Liq. Cryst., 163, 157-162). PA0 (a) Sufficient adhesion to glasses, metals, and transparent conductors (typically mixtures of indium-tin oxides(ITO) or tin oxides) so that the alignment film remains relatively homogenous in covering the substrate, i.e., not rubbed off or cracked by the mechanical energy supplied during the buffing step. PA0 (b) Sufficiently high thermal stability such that subsequent steps in the fabrication of the LCD device, such as lamination of the plates or sealing of the LCD cell do not cause either decomposition of the film or loss of the unique optical direction. PA0 (c) Sufficiently high thermal glass transition temperature, Tg, such that thermal steps which may be necessary in LCD fabrication do not result in loss of the rubbing direction or large induced stresses. Likewise, a high glass transition means that the alignment film will be less likely to melt or deform under heating by the friction caused by the buffing operations. PA0 (d) Sufficient strength in very thin layers (ranging from hundredths of microns to microns) such that the film remains continuous during buffing and such that any subsequent step taken during the processing does not cause either loss of adhesion or breaking of the film due to stresses. PA0 (e) Sufficient inertness, in the chemical sense, so that the alignment film does not behave adversely to the liquid crystal molecules or various agents which may be used to seal the finished display. PA0 (f) Sufficient inertness or stability, in the photo-chemical sense, so that the alignment film does not decompose under light which might need to be transmitted through it or from intense light which may strike it during its intended use. PA0 (g) Sufficient inertness, in the electrochemical and electromotive senses, so that the alignment film does not react with the Liquid Crystal charge under electrolysis conditions or so that the film is not changed by the electric field that must be used to operate the device.
The tilt angle and its magnitude are very important in the various electro-optic responses of the LCD device. The stability, legibility, and reliability of the LCD are all related to the magnitude of the tilt angle and to the unchanging nature of this magnitude. It is very important that the polymer film have the following properties:
When the above properties of the polymers are maximized, the range of conditions under which they can be used in manufacturing of LCD's is enhanced. Thus, wide latitude is given to the fabrication process, an this, as is understood by those skilled in the fabrication of LCD's, is highly desirable.
Although a number of coupling agents and surfactants may be used to modify the tilt angle, the nature of the polymer is by far more important. Of all the polymer classes or small molecules known to show alignment behavior, polyimides are preferred. It can also be stated that within the class of polymers denoted by polyimides, that many different approaches have been taken with the aim toward finding structures which fulfill the requirements and allow wide ranges of the tilt angles to be realized in practical manufacturing of LCD's.
The repeating structure of polyimides, which can be denoted by ##STR1## wherein, A is one or more residues from an acid dianhydride group and B is one or more residues from diamine compounds actually relies on long chain or strand of the type (--ABABABAB--)n where n is very large. Changes in the intrinsic nature of the polyimide, i.e., those properties which relate to the factors cited above, may be altered by use of certain structures in either A or B above. However, the art of changing the properties of polyimide surfaces and/or controlling the magnitude of the tilt angle while maintaining the intrinsic properties (a)-(g) above is rather difficult. The need to maintain high values of the molecular weight while keeping electronic, spatial, tilt angle magnitude, and/or surface properties under control has been very difficult.
Generally, properties (b), (c), and (d) above are maximized by the use of aromatic-ring (Ar) groups in both the A and B side of the polyimide. In so doing, this also tends to maximize the other properties. When all the alignment film intrinsic properties are at their highest values, the process latitude is highest.
The films of polyimides used to control the tilt angle of liquid crystal molecules in all types of liquid crystal displays (LCD's) are very thin, 100-2000 .ANG.. In a complex process, the tilt is induced in a unique direction of the polymer by gentle buffing with specific cloths. Twisted Nematic LCD's, such as those found in pocket TV sets and watches, need low tilt angles in the range of 1-5 degrees. Super Twisted Nematic (STN) and super twisted birefringent effect (SBE) LCD's, both of which require higher tilt angles, are slower switching devices, typically between 6-25 degrees. The actual tilt angle obtained is a function of polymer ordering on the surface (analogous to cold working of metals), the resulting surface energy (an intrinsic property), the nature of the cloth used to buff the surface (an intrinsic property of its fibers), and the amount of buffing pressure (an extrinsic process variable). In addition to these variables, each of the hundreds of commercial liquid crystal formulations probably interacts differently with a given surface. In general, however, the single most important factor determining the value range of the tilt bias angle is the intrinsic character of the polymer used to control this angle.
Although there is a plethora of patent references dealing with fluorinated polyimides, which are becoming more and more popular, due to their excellent properties and fitness for this application, none of these references has recognized the importance of the length of pendant perfluorinated chains, which have to contain more than 6 perfluorinated carbon atoms, and preferably 8 or more, in order to be effective in raising the tilt bias angle.
U.S. Pat. No. 4,749,777 (Kohtoh et al), issued Jun. 7, 1988, describes a liquid crystal aligning agent consisting essentially of a polyimide resin obtained by the polymerization of a diamine, a tetracarboxylic acid dianhydride and a mono-amine. It is claimed that the addition of the aliphatic mono-amine species can cause high tilt angles. However, use of aliphatic mono-amines in the polyimide structure reduces thermal decomposition temperatures. Use of the aliphatic mono-amine also lowers the molecular weight which results in reduction in strength, as well as in reduction of the glass transition temperature, which is related to the molecular weight. It is seen that these properties are undesirable as mentioned in (a)-(d) above.
U.S. Pat. No. 4,735,492 (Sekine et al), issued Apr. 5, 1988, describes a liquid crystal display device containing a liquid crystal orientation controlling film made from a fluorine-containing polyimideisoindoloquinazolinedione. The device is claimed to have high tilt angle and excellent viewing properties.
U.S. Pat. No. 4,864,008 (Murata et al), issued Sep. 5, 1989, describes an invention which provides diamino compounds having pendant hydrocarbon groups, and liquid crystal aligning films comprising polyimides which are obtained from the diamino compounds. Since, however, the pendant groups contain long chains of --CH.sub.2 --CH2--, their ultimate thermal stability is considerably lower; generally speaking, aliphatic species, which contain at least two methylene groups (--CH.sub.2 --CH.sub.2 --) in a side by side arrangement, decompose by elimination of hydrogen (H.sub.2) and olefins (commonly called beta-hydride elimination). These thermal decompositions are usually below about 250.degree. C. Also, it can be recognized that their glass transition temperatures are also much lower. Several species of this disclosure, were claimed to change tilt with the number of rubbing cycles, and this implies that the inertness of the polymer is not great with respect to mechanical energy being put into the film by the rubbing process, clearly a narrowness in process latitude, which is quite undesirable from a manufacturing perspective.
Japanese Patent Application Publication 2-4225 (Application No: 63-153134) (Minamizawa et al), publication date Jan. 9, 1990, describes a process for the manufacture of a liquid display element comprising heat treating a liquid crystalline orientation film of a polyimide obtained by treating a dicarboxylic acid anhydride with a fluorine-containing diamine, under a condition of liquid crystals in contact therewith to a temperature at least as high as the liquid transition temperature of said liquid crystal.
Japanese Patent Journal Kokai Patent No. HEI 1[1989]-180860 (Application No.: Sho 63[1988]-4760) (Matsuura et al), publication date Jul. 18, 1989, describes an aromatic diamine containing pendant groups of the formula --O--(CH.sub.2).sub.n (CF.sub.2).sub.m F, in which n is 1 or 2 and m is 0 to 10.
Japanese Patent Journal Kokai Patent No. SHO 63[1988]-259515 (Application No.: Sho 62[1987]-94180) (Mukatomi et al) publication date Oct. 26, 1988, Japanese Kokai Patent No. Hei 1(1989)-180518 (Application No.: Sho 63(1988)-4365) (Hisatomi et al), publication date Jul. 18, 1989, and Japanese Kokai Patent No. Hei 1(1989)-180519 (Application No.: Sho 63(1988)-4366) (Hisanagi, et al), publication date Jul. 18, 1989, describe liquid crystal display device orienting agents, which consist of polyimide resins, which contain perfluoroalkyl groups containing 1-6 perfluorinated carbon atoms.
Japanese Patent Application Publication Kokai: Sho 63-313124 (Application No.: Sho 62-149006) (Yokokura et al) publication date Dec. 21, 1988, describes a liquid crystal display element comprising a polyimide orientation film, which is the reaction product of a diamine compound having long chained hydrocarbon groups, a diamine compound having trifluoromethyl groups and a tetracarboxylic dianhydride.
Japanese Kokai Patent No. Hei 2(1990)-4224 (Application No.: Sho 63(1988)-153133) (Hitachi et al) publication date Jan. 9, 1990, describes a method for manufacturing a liquid crystal display device wherein a liquid crystal is heated at a temperature equal to or higher than the liquid transition point of said liquid crystal while said liquid crystal is being contacted with a liquid crystal orientation film consisting of a fluorine atom containing polyamide polymer.
Japanese Kokai Patent No. Sho 62(1987)-127827 (Application No. Sho 60(1985)-270009) (Sato et al), publication date Jun. 10, 1987, and Japanese Kokai Patent No. Sho 62(1987)-87939 (Application No. Sho 60(1985)-228236) (Sato et al), publication date Apr. 22, 1987, describe compositions for liquid crystal oriented films which contain a polyamide acid or polyimide, respectively, and at least on of the acid or diamine contains fluorine atoms.